It is known in connection with mobile radio systems that, besides a main base station, also one or several additional base stations, i.e. infill base stations, may be positioned in one radio cell. An infill base station may be separate or it may be based on using an antenna and/or transceiver units of the main base station. It is reasonable to use infill base stations when frequency allocation plans and schemes and shortage of frequency range make it necessary to reuse the same traffic channel in the cells of a mobile radio system, such as a cellular radio system, at shorter distance than would be done according to normal frequency allocation plans. The infill base stations have smaller coverage areas, because the traffic between them and subscriber stations use in both directions lower radio powers than the normal main or mother base stations do.
Those traffic channel units of one infill base station or the main base station which are allocated to be used by infill base stations can use a group of infill base station channels. The channels of an infill base station are conventionally configurated in such a way that the infill base stations use the control channel of the main base station. The infill base stations thus monitor signalling of subscriber stations occurring on the control channel of the main base station. Then the subscriber stations, i.e. radio telephones, desiring a connection with the network, contact on the control channel of the main base station an infill base station, which transmits a request for establishing a connection to an exchange of the mobile radio system. The exchange then allocates some suitable traffic channel of the additional, i.e. infill base station for the traffic between the subscriber station and the infill base station, and a command to move onto this traffic channel is transmitted to the infill base station, which starts monitoring that traffic channel.
A problem with allocating a traffic channel according to the prior art is that, when the exchange of the mobile radio system tells an infill base station to start monitoring the traffic occurring on a predetermined control channel, the infill base station actually starts monitoring this control channel and searches for signalling of subscriber stations on that channel. An attentive reader notices that, in a case according to the prior art, the infill base station itself does not send any signalling to the main base station on the control channel. If now that the infill base station monitors the control channel of the predetermined main base station, that main base station stops functioning abruptly, for instance if its transceiver is damaged, all subscriber stations, i.e. radio telephones, within the coverage area of that main base station begin, after a process of searching for a control channel, to monitor the control channels of the other main base stations located adjacent to the main base station mentioned above. Then the infill base stations within the coverage area of said main base station are left monitoring the "idle" control channel, on which the subscriber stations do not signal any longer. The infill base station is then entirely worthless and of no use.